Optical assembly

ABSTRACT

An optical assembly comprises an optical device ( 103 ); an enclosure ( 101 ) for containing the optical device ( 103 ), the enclosure ( 101 ) including a transparent section ( 107 ) to allow passage of a light beam ( 104 ) to or from the optical device ( 103 ); and means for measuring the attenuation of a test light beam ( 212 ) through the transparent section( 107 ). Preferably, the optical assembly also comprises means for compensating for the measured amount of attenuation by adjusting a measurement or a characteristic of the optical device ( 103 ) or a related optical device to compensate for the measured amount of attenuation.

FIELD OF THE INVENTION

The present invention relates to an optical assembly, particularly foruse in an industrial environment. The optical assembly preferablyincludes an enclosure for containing an optical device, the opticaldevice typically being either a light source or a detector, and theenclosure includes a transparent window to allow light to enter or leavethe enclosure to or from the optical device.

Industrial environments are often characterised by noise, vibration,temperature, and humidity, and devices used in industrial environmentsare exposed to fluids, solvents, airborne dust and vapour. It issometimes desirable to install sensitive equipment in such environments,for example, for the purposes of process monitoring. Depending on themode of measurement, some or all of the industrial environmental factorsmay easily be screened or eliminated from interfering with the sensitiveequipment.

In particular, optical sensors are commonly used for process monitoring.For instance, in one application disclosed in WO 01/66352 a light sourceis used to illuminate a test surface, and characteristics of thebehaviour of the light resulting from interaction with the surface aremeasured and quantified by means of optical detectors.

In this situation of optical sensing, elimination of interference fromthe industrial environment presents some particular challenges. Locationof an optical device within an enclosure will prevent ingress ofcontaminants into the device, but the optical signal or beam must passthrough some part of the enclosure in order to interact with the device.A transparent section must therefore be included in the enclosure.However, contamination still builds up on the outside of the transparentsection, and may impede the passage of light into or out of theenclosure and adversely effect the measurement. Particularly inunattended operation, there will always be uncertainty regarding thepresence and/or the extent of contamination present and this maycompletely negate any benefit of using the instrument.

SUMMARY OF THE INVENTION

According to the present invention, an optical assembly comprises:

an optical device;

an enclosure for containing the optical device, the enclosure includinga transparent section to allow passage of a light beam to or from theoptical device; and

means for measuring the attenuation of a test light beam through thetransparent section.

The optical assembly of the present invention has the capacity tomeasure and thereby monitor the attenuation caused by any contaminationthat has built up on the transparent section of the enclosure.

The measurement may be used to merely alert an operator when theattenuation reaches a threshold value, indicating that the contaminationbuild up has reached a level where it may be adversely effecting theoperation of the optical device. However, preferably the opticalassembly includes means for compensating for the measured amount ofattenuation. Preferably, a measurement or a characteristic of theoptical device or a related optical device is adjusted to compensate forthe measured amount of attenuation. If the optical device is a detector,the sensitivity of the detector may be adjusted or the signal from thedetector may be adjusted by an appropriate factor. If the optical deviceis a light source, the brightness of the light source may be adjusted.Rather than adjusting a characteristic of the optical device itself, acharacteristic or measurement of an associated device may be adjusted.If the device is part of a source-detector pair, it may be that theother device is adjusted. For example, if the optical device is asource, the signal from the associated detector may be adjusted tocompensate for the attenuation of the light beam from the source.

Preferably, the light path to or from the optical device and the testlight path intersect substantially at the transparent section. Thisensures that the contamination of the transparent section is measured atthe same position as the position where the beam from or to the opticaldevice passes through the transparent section.

Preferably, the optical assembly includes a light source and a detectorfor generating and detecting the test light beam respectively, the lightsource and the detector being located on opposite sides of thetransparent section. Preferably, the light source is located externallyto the enclosure and the detector is located inside the enclosure.

Preferably, the transparent section is recessed into the enclosure. Thishelps to prevent airborne dust and other contaminants from reaching andbuilding up on the transparent section. Preferably, the transparentsection is located in an enclosed passage, and the light path throughthe transparent section passes along the length of the passage.

Preferably, when the test light beam is generated by a test source anddetected by a test detector, the component which is located outside theenclosure is recessed. This prevents contamination from building up andaffecting the performance of the external component. Preferably, theexternal component is located in a passage through which the test beampasses.

In a preferred aspect of the present invention, the optical assemblyincludes means for directing a flow of gas onto an external surface ofthe transparent section, to reduce build up of contaminants on thesurface. Cleaning fluid may be introduced in the flow of gas.

In this preferred aspect of the present invention, the flow of gasdirected at the external surface of the transparent section serves tosubstantially reduce build up of contaminants on the surface which wouldotherwise attenuate the emitted or received beam, and adding cleaningfluid to the flow cleans away any residue which has built up.

Preferably, the flow of gas is compressed gas, and more preferablycompressed air.

Preferably, the flow of gas is directed along the passage in which theexternal component of the test source-detector pair is located, and theflow is directed away from the external component and is incident on thetransparent section. Furthermore, the flow of gas along the passage awayfrom the test light source or detector also prevents build up ofcontaminants on the test source or detector.

In one embodiment, wherein the optical device is a light source, theoptical assembly preferably includes a beam splitter located inside theenclosure and arranged to direct a portion of the beam into a testdetector which is also arranged to detect the test beam. This enablesthe test detector to also be used to measure the output of the lightsource, to enable a final measurement result to be adjusted forvariations in the light output of the source, as well as being adjustedto allow for attenuation of the beam by contamination of the transparentsituation.

In another embodiment, wherein the optical device is a detector, thedetector can be arranged to detect both the test beam and a primarybeam. When the light source for the primary beam is deactivated, and thelight source for the test beam is activated, the detector receives lightintensity from the primary beam, and vice versa. The advantage of thisconfiguration is that it is simpler and requires fewer components.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings, in which;

FIG. 1 is a schematic drawing of a first embodiment of the presentinvention;

FIG. 2 is a schematic drawing of a second embodiment of the presentinvention in which the optical device is a light source;

FIG. 3 is a schematic drawing of a third embodiment of the presentinvention;

FIG. 4 is a schematic drawing of a fourth embodiment in which theoptical device is a detector; and

FIG. 5 is an elevated view of an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a sealed enclosure 101 containing an optical device103. The enclosure 101 is constructed to meet one of the most stringentrequirements for electrical enclosures (e.g. IP67) including therequirement of being able to be totally immersed in water withoutleakage.

The enclosure 101 is positioned relative to a surface 102 to bemeasured. Optical device 103 represents, in one embodiment, an opticalsource e.g. LED or laser etc, or in another embodiment an opticaldetector. A light pathway 104 is shown to describe the motion of lighteither (a) from the source 103 to the surface 102 or vice versa in thecase of the device 103 being a detector. The light beam travels along apassage 105 that recesses the window 107 into the enclosure 101. Otheroptical elements such as filters, polarisers, lenses etc may be locatedbetween the optical device 103 and the enclosure window 107. Sealing ofthe enclosure 101 is accomplished using a window 107 that is attachedinto the enclosure 101 in such a manner as to exclude contamination e.g.by means of an O-ring seal. Compressed gas is directed onto the externalsurface of the window 107 through passage 108. The compressed gas issupplied from an external supply at a controlled pressure 109. Thecompressed gas, having flowed across the window surface, travels alongthe passage 105 and is exhausted to ambient 110. Bursts of cleaningfluid may be introduced upstream in the compressed gas stream so that ittravels down passage 108 and is applied to window 107 and cleans awayany residue on the window 107.

FIG. 1 illustrates an arrangement for the diagnosis of windowcontamination. Gas tube 108 accommodates a test light source 211 thatprojects a test beam along gas tube 108 onto and through window 107. Thetransmitted light continues to follow path 212 and finally falls ontotest detector 213 where it is converted into an electrical signal. Lightpath 212 is configured to intersect light path 104 substantially at thewindow 107.

The method of contamination diagnosis involves initial calibration ofthe signal from test detector 213 when test light source 211 isactivated and when window 107 is in a contamination-free state. This ispreferably conducted at or shortly after manufacture, but beforeinstallation in the contaminating environment. The signal from testdetector 213 in the contamination-free state is measured and recorded byelectronic systems associated with the optical device 103—this value iscalled ID₀. Following the installation of the optical device 103 in thepotentially contaminating environment, a similar operation of measuringthe signal from test detector 213 is conducted to give a value calledID_(t). Test light source 211 is only activated when test detector 213is being measured; for the rest of the time it is off. If contaminationhas been able to attach to the external surface of window 107, such thatit impedes primary light path 104, then it will also impede test lightpath 212 which will result in less light intensity falling on testdetector 213 and give rise to less electrical signal ID_(t). Acomparison of ID_(t) with the stored value ID₀ will determine whetherID_(t) has decreased as a result of contamination on window 107.

In order to accommodate test light source 214, the gas flow isintroduced into tube 108 via a T-junction 214. The advantage of thisconfiguration is that test light source 211 is located at a positionseparated from the contaminating environment by a tortuous path as wellas being protected by a gas flow opposing the ingress of contamination.Positioning light source 211 at this position significantly reduces theprobability of contamination.

In a further refinement of the invention, the sensor system may be madeto be substantially tolerant to moderate levels of contamination onwindow 107. Assuming the properties of light paths 212 and 104 areattenuated by contamination in substantially similar extents, then thecontamination induced attenuation in test light path 212 may be used asan estimate for the attenuation in primary light path 104 caused bycontamination on window 107. Attenuation in test light path 212 can beenumerated as: ${Attenuation} = {\frac{{ID}_{t}}{{ID}_{0}}.}$

Using this value as an estimate for the attenuation in primary lightpath 104, a correction can be applied to the measured signal fromdetector 103. Thus:${IS}_{Ct} = \frac{{IS}_{t} \times {ID}_{0}}{{ID}_{t}}$

Where IS_(t) is the measured signal from detector 103 at or aftermeasurement ID_(t), and IS_(Ct) is a corrected value for detector 103,based on the estimated effect of contamination.

If optical device 103 is a source, the measured signal from anassociated detector may be adjusted, or the brightness of the source maybe adjusted.

In a preferred method, ID_(t) will be measured frequently perhaps daily,hourly or even more frequently depending on the probability ofcontamination and its impact on the measurement efficacy or downstreamuse.

The embodiment shown in FIG. 1 depicts a configuration in which themeasurement light path 104 makes an angle of approximately 60 degreeswith the normal of the measurement surface 102. It will be recognised bya person skilled in the art that the principles described are notlimited to this configuration and that any angle can be accommodated. Itwill also be recognised that a multiplicity of angles and configurationsmay be combined in a single instrument or enclosure.

As illustrated in FIG. 2, the addition of a beam splitter 315 into theconfiguration already shown in FIG. 1 enables the output of a primarylight source 103 to be measured. This maybe important in applicationswhere measurement accuracy requirements are beyond the stabilityspecifications of the light source output. FIG. 2 shows a beam splitter315 inserted into the same configuration as FIG. 1. Much of the commonlabelling has been omitted for clarity. In the instance in which 103 isa light source, the output light path 104 travels from the source 103towards the measurement surface 102. When it passes the beam splitter315 a small proportion of the light intensity is reflected along path316 and impinges on test detector 213 where it is converted into anelectrical signal. The electrical signal may be used as a diagnosticmeasure by comparing periodic measurements with a stored initial valuesuch that gradual or sudden changes in light output may be identifiedand an error signal or alarm triggered. Alternatively (or additionally)the electrical signal may be used to normalise the results of anysubsequent optical measurement for which the light source 103 supplieslight intensity e.g. the measurement of an associated detector. Smallvariations in light output of source 103 may, in this way, be correctedand their effects substantially eliminated from a final measurementresult.

FIG. 3 shows an embodiment of a single optics tube in which all of thepreviously described features have been combined. In order to operatethe full range of features possible, the light sources 103 and 211 mustbe individually controlled. In order to measure window contaminationtest light source 211 must be activated and 103 deactivated. In order tomeasure the light output of primary light source 103, primary lightsource 103 must be activated and test light source 211 deactivated.

If the physical configuration permits, a simplified embodiment of thecontamination diagnostic device may be employed as depicted in FIG. 4.The sensor detector 417 is used to receive the measurement signal alonglight path 104, but when the light source for light path 104 isdeactivated and test light source 211 is activated the detector 417receives light intensity from the direction of test light path 212. Bothlight paths travel through the optical element 107 that seals theinstrument's enclosure 101. Therefore, the use of test light path 212can be used in order to measure the presence and degree of contaminationon optical element 107. The same method of correcting for contaminationon optical element 107 can be used as previously described. Test lightsource 211 and test light path 212 travel along tube 108 (as describedpreviously) and are protected from the possible ingress of contaminationcoming up tube 105.

The advantage of this configuration is that it is significantly simplerthan the embodiments that have preceded. However, it can only be used insituations in which detector 417 is capable of measuring light intensityand also where the geometric configuration permits both light paths 212and 104 to be incident on a single detector 417. It will be recognisedby one skilled in the art that FIG. 4 represents only one of manygeometric configurations that satisfy these criteria.

FIG. 5 shows an actual embodiment, having a source assembly fordirecting a beam of light at a surface, and two detector assembliesarranged at different orientations to detect light scattered andreflected from the surface, respectively.

The source assembly has the arrangement shown in schematic FIG. 2, andincludes a source 103 located in an enclosure (not shown) having atransparent window 107 for allowing passage of the beam emitted by thesource 103. The beam emitted from the source 103 passes along passage105 to be incident on a surface. Inside the enclosure (not shown),between the window 107 and the source 103 is a beam splitter 315 whichdirects a portion of the emitted beam on to a detector 213. A testsource 211 is located at the end of a recessed passage (not shown) todirect a test beam through the window 107 to be incident onto detector213. When the source 103 is switched on, detector 213 receives a portionof the emitted beam from the beam splitter 315, and when the source 103is switched off, the test source 211 is switched on, and the detector213 detects the test beam. A flow of air A passes along passage 214 tojoin the passage in which the test source 211 is located at aT-junction. A flow of air thus is directed away from test source 211 andis incident on window 107.

Both detector assemblies have the arrangement shown in schematic FIG. 4,wherein the same detector 417, 517 is used to detect incident radiationfrom the surface, and the test beam is generated by a test source 311,411. Each detector 417, 517 is located in an enclosed chamber 201, 301such that a detecting surface of the detector 417, 517 is adjacent awindow 207, 307. The window 207, 307 is recessed such that incominglight passes along a passage 205, 305 to be incident on the window 207,307. A test source 311, 411 directs a test beam at the window 207, 307along a passageway 208, 308. The test beam passes through the window207, 307 and is detected by the detector 417, 517. A flow of air B, C isdirected into the passage 208, 308 from a side passage 314 such that theair flow is directed along passage 208, 308 away from the test source311, 411 and is incident on the window 207, 307.

For the purposes of this specification it is to be clearly understoodthat the word “comprising” means “including but not limited to”, andthat the word “comprises” has a corresponding meaning.

Modifications and variations as would be apparent to a skilled addresseeare deemed to be within the scope of the present invention.

1. An optical assembly comprising: an optical device; an enclosure forcontaining the optical device, the enclosure including a transparentsection to allow passage of a light beam to or from the optical device;means for measuring the attenuation of a test light beam through thetransparent section; and means for compensating for the measured amountof attenuation.
 2. An optical assembly according to claim 1, wherein themeans for compensating for the measured amount of attenuation comprisesmeans to adjust a measurement or a characteristic of the optical deviceor a related optical device to compensate for the measured amount ofattenuation.
 3. An optical assembly according to claim 1, arranged suchthat the light path to or from the optical device and the test lightpath intersect substantially at the transparent section.
 4. An opticalassembly according to claim 1, wherein the transparent section isrecessed into the enclosure.
 5. An optical assembly according to claim4, wherein the transparent section is located in an enclosed passage,and the light path through the transparent section passes along thelength of the passage.
 6. An optical assembly according to claim 1,including a light source and a detector for generating and detecting thetest light beam respectively, the light source and the detector beinglocated on opposite sides of the transparent section.
 7. An opticalassembly according to claim 6, wherein the source or detector which islocated outside the enclosure, is recessed.
 8. An optical assemblyaccording to claim 7, wherein the external component is located in apassage through which the test beam passes.
 9. An optical assemblyaccording to claim 8, and further comprising means for applying a flowof gas directed along the passage away from the external component, andis incident on the transparent section.
 10. An optical assemblyaccording to claim 1 and including means for directing a flow of gas tobe incident on an external surface of the transparent section to reducebuild up of contaminants on the external surface.
 11. An opticalassembly according to claim 10, wherein the flow of gas is compressedgas.
 12. An optical assembly according to claim 1, wherein the opticaldevice is a light source, and the optical assembly includes a beamsplitter located inside the enclosure and arranged to direct a portionof the beam from the light source into a test detector which is alsoarranged to detect the test beam.
 13. An optical assembly according toclaim 1, wherein the optical device is a detector, and the detector isarranged to detect both the test beam and a primary beam.
 14. An opticalassembly according to claim 8 and further comprising (a) means fordirecting a flow of gas to be incident on an external surface of thetransparent section to reduce build up of contaminants on the externalsurface, (b) the mans for directing a flow of gas including a compressedgas source, (c) the optical device is a light source, and the opticalassembly includes a beam splitter located inside the enclosure andarranged to direct a portion of the beam from the light source into atest detector which is also arranged to detect the test beam.
 15. Anoptical assembly according to claim 14, and wherein the optical deviceis a detector, and the detector is arranged to detect both the test beamand a primary beam.